When you choose "Trigger based on Audio" a little box wil appear next to the BPM and VLJ button that will show you the wave form of your incoming audio. If the box appears with no "line" in it you need to right click on the box to see the sound cards available to QuickShow.

If you are running Windows Vista or 7 you may not be able to get access to the audio without digging around in the Windows Sound options. Out of 4 computers, I have about a 50% success rate getting access to the sound card in Window Vista and 7. (God Bless Windows XP...)